Optical fibers have become the backbone of terrestrial telecommunications systems, due to their enormous information carrying capacity. An optical fiber is a substantially transparent, flexible pipe, typically about the same diameter as a human hair, which can guide optical energy (i.e., light) along a path. Normally, an optical fiber is made of either glass or plastic.
An optical fiber guides light within a core that is surrounded by a cladding that serves to confine light within the core via a process referred to as “total internal reflection.” In other words, as the light travels down the fiber, the interface between the core and the cladding acts like a mirror that reflects light rays back into the core material—even at bends in the optical fiber (subject to some practical limits). As a result, the optical fiber can guide light along a significantly circuitous route with little attenuation of the light.
Optical fibers typically fall into two main categories—single-mode fibers and multimode fibers. In a single-mode fiber, all of the optical energy of a light signal propagating through the fiber travels substantially along a single path down the core of the fiber. As a result, all of the optical energy in the signal travels the same distance and emerges from the output facet of the fiber concentrated in a single spot (i.e., as a single optical mode).
In a multimode fiber, however, optical energy of the light signal can take many different paths (typically hundreds to tens of thousands) down the core of the fiber. The different optical modes supported by the fiber reflect from the core/cladding interface at different angles and, therefore, travel different total distances through the fiber. The different modes can emerge from the output facet of the fiber with slightly different phase, launch angle, and from different locations on the output facet. In telecommunications applications, the disparity between the exit characteristics of the modes conveyed by a multimode optical fiber normally limits their use to short-distance applications, while single-mode fibers are primarily used for long-distance links.
In addition to their use in telecommunications, optical fibers have found utility in medical imaging applications. Conventional confocal microscopes or multiphoton microscopes are limited to imaging biological tissue to depths much less than one millimeter. Optical fiber-based endoscopes, on the other hand, can be inserted directly into a subject, such as an animal or a human, to enable imaging of matter deep within tissue. Endoscopes are widely used to image region within large body cavities, such as the gastrointestinal tract, the respiratory tract, as well as other regions such as brain matter and joint tissue during arthroscopic surgery.
A typical conventional flexible endoscope includes a bundle containing thousands of optical fibers, a high-power light source, and a miniature camera. Some of the fibers in the fiber bundle are used to channel light to the objective end to illuminate the tissue of interest. The remaining fibers in the bundle are used to relay optical images from the sample end to the camera.
More recently, endoscopic imagers have begun incorporating a scanning function, wherein a scanner at the source end is used to illuminate each fiber in the fiber bundle sequentially. This enables the output beam from the fiber bundle to be scanned over a sample area. The reflected light (or fluorescence signal) from the sample is then imaged back through the same fiber core so that a full sample image can be developed over a single scanning period.
One such fiber-bundle-based scanning endoscope was disclosed by French, et al., in U.S. Patent Application Publication 20110137126, published Jun. 9, 2011. In this system, a synthesized curved wavefront is provided to the input end of a fiber bundle containing thousands of single-mode optical fibers. By controlling this wavefront, the relative phase of the light in each single-mode optical fiber can be made to constructively interfere in a desired manner at the output end of the fiber bundle. Control over this constructive interference enables focusing of the light emerging from the fiber bundle over different points in a three-dimensional volume.
Unfortunately, due in part to the limited number of optical fibers in the fiber bundle, the image quality of such endoscopes is limited. In addition, the relatively large diameter of the fiber bundle (0.5 mm or more) makes them incompatible for insertion into sensitive, confined areas of tissue. As a result, efforts toward reducing the size of these imaging systems have been of great interest.
Di Leonardo, et al., presented a fiber-based imaging system based on a single optical fiber in “Hologram transmission through multi-mode optical fibers,” Optics Express, Vol. 19, No. 1, pp. 247-254 (2011). The disclosed imaging system replaced control of the optical modes in each single-mode fiber of a bundle of optical fibers by control over the different optical modes travelling through a solitary multi-mode optical fiber. As a result, the disclosed imaging systems can potentially have a significantly reduced size. Unfortunately, these systems are limited to scanning the output beam of the optical fiber in two dimensions. Further, the reliance of these single-fiber systems on a collimating lens at the output of the optical fiber places an upper bound on which the size of such systems can be reduced.
A small diameter, fiber-based beam scanning system that can scan an output signal in three-dimensions, therefore, remains unrealized in the prior art.